The invention relates generally to operation of gas turbines and more specifically to control of fuel supplied to the gas turbine during transition between turbine operating modes.
Industrial and power generation gas turbines have control systems with controllers that monitor and control their operation. These controllers govern the combustion system of the gas turbine, controlling various modes of operation from cold startup through base load. In addition to operating the gas turbine to maintain safe and effective base load operation while avoiding combustion boundaries, the controller must maintain gas turbine emissions compliant with government regulations and contractual obligations and at the same time promote efficient power output.
The control functions of the controller may vary between different types of gas turbines but may include functions related to startup control, acceleration control, speed control, generator load control, compressor control, exhaust temperature control, inlet guide vane control, emissions control and fuel control, among others. Fuel control is a reference from the controller and feedback of the fuel control valves. A fuel demand reference (sometimes referred to as fuel stroke reference) is determined by various turbine parameters such as speed and temperature as called for in various the modes of turbine operation.
The startup of the gas turbine includes transition through a number of stages associated with increasingly higher load and operating temperatures, as fuel staging is modified. Table I illustrates typical modes of fuel staging for one type of gas turbine by General Electric.
TABLE IMODES OF FUEL STAGINGMODE 1MODE 2MODE 3MODE 4MODE 5MODE 6
Fuel flow scheduling may include requirements for a combustion reference temperature achieving designated values appropriate to a particular fuel scheduling/burner operation. Plant control algorithms may also enforce this operation within designated boundaries for combustion reference temperature and other parameters.
Traditional gas turbine controls assume constant combustion efficiency throughout the different modes of gas turbine operation. However, in transient transfers between gaseous combustion modes and certain low-load operation points, it is possible for combustion efficiency to change quickly in a short period. This change causes several problems in the operation of a combustion turbine, including failed gaseous mode transfers and oscillations in fuel control. Accordingly, it would be desirable to provide systems and methods to avoid such problems during mode transfers.